Fluids preferentially migrate into highly permeable zones in subterranean formations. This migration is undesirable when injecting treatment fluids into hydrocarbon-bearing formations for post-primary recovery of residual hydrocarbons. The treatment fluids channel through the highly permeable zones bypassing the less permeable zones. The result is poor conformance and flow profiles of the treatment fluid in the formation. The hydrocarbons residing in the less permeable zones are not produced and the overall yield of hydrocarbons from the formation is reduced.
Highly permeable zones in subterranean formations are plugged to prevent migration of treatment fluids into them and to divert treatment fluids into adjacent less permeable zones. One plugging method is to inject fluids into the formation prior to treatment fluids. The injected plugging fluids preferentially enter and plug the highly permeable zones. Subsequently injected treatment fluids are diverted into less permeable zones facilitating recovery of the hydrocarbons therein.
It can be desirable to prevent migration of fluids in subterranean porous media other than hydrocarbon-bearing formations, such as soils. Plugging fluids are injected into soil as stabilizers to reduce soil permeability. Soil stabilizers seal subterranean building structures to prevent migration of water into the structure. Soil stabilizers also prevent seepage of water through permeable water retainers such as irrigation ditches and earthen dams.
A number of processes are known for reducing the permeability of soils. U.S. Pat. No. 4,291,069 to Pilny teaches a process for plugging a porous subterranean formation with an aqueous gel composed of an amide polymer and a water-soluble polyaldehyde. The polymer and polyaldehyde are mixed at an alkaline pH. They react to form a gel within the formation after a short time at ambient temperature. U.S. Pat. No. 3,495,412 to Sakata et al teaches a process for stabilizing a water-permeable soil. A three-component aqueous solution comprising urea, formaldehyde and polyvinyl alcohol is injected into a water-permeable soil where it is cured with an acidic substance.
Polymer plugs are used specifically in hydrocarbon recovery processes. U.S. Pat. No. 4,289,203 to Swanson teaches an oil displacement method using a shear-thickening composition which contains water, a high molecular weight polyalkylene oxide and a phenolformaldehyde synthetic resin. The composition is a liquid in its quiescent state, but assumes a gel-like character upon agitation. The gel is useful as a water diversion agent.
U.S. Pat. No. 4,300,634 to Clampitt teaches using a polymer in conjunction with a steam injection process. A fluid is injected into a hydrocarbon-bearing formation prior to steam treatment. The injected composition contains aldehyde and phenol gelling agents, a polymer, a surfactant, and water. Once in place the polymer prevents the surfactant from foaming until the steam treatment begins. However, the polymer breaks down under the high temperature of the steam treatment allowing the surfactant to foam. The resulting foam plugs the more permeable zones of the formation and directs the steam into the less permeable zones. The foam utilizes steam as the gas phase and is self-collapsing upon termination of the steam treatment.
Partially hydrolyzed polyacrylamide is gelled in situ to reduce the permeability of highly permeable zones in a hydrocarbon-bearing formation prior to fluid treatment. Partially hydrolyzed polyacrylamide and a cross-linking agent, a polyvalent metal such as chromium or aluminum, are injected into the formation. The gel sets in the more permeable zones of the formation. The concentration of cross-linking agent determines the gelation rate.
The gels taught by the above-cited prior art processes tend to be thermally unstable in a hydrocarbon-bearing formation environment. As such, the gels provide only temporary plugs of hydrocarbon-bearing formations. They are particularly disadvantageous for high temperature steam injection processes. Furthermore, the in situ gelation time of the prior art gels is difficult to control.
A need exists for a process to selectively prevent the migration of fluids into highly permeable zones of a subterranean formation under elevated thermal conditions. A process is needed, which injects a fluid into the highly permeable zones and then reacts the fluid in situ under formation conditions to produce a non-flowing, water insoluble plug. A process is needed to provide a rigid and permanent plug of the selected highly permeable zones.